An existing driving circuit that drives a light emitting element such as a semiconductor laser has been known. For example, a light emitting element driving circuit has been known in which a switch circuit turning ON/OFF in accordance with an input data signal and a current source supplying a driving current to a light emitting element when a switch is turned ON are connected in series.
For example, in a single end-type driving circuit formed by combining an emitter follower and an emitter grounding circuit, a current source may not be installed on the ground side of the emitter grounding circuit. Therefore, there is a problem that resistance to voltage variation of a power source of the driving circuit is low and a driving signal is deteriorated due to occurrence of the voltage variation of the power source.
By contrast, the configuration in which a differential-type driving circuit is employed and a current source is installed on and ground side of the emitter grounding circuit of a differential pair may also be considered. In the differential-type driving circuit, for example, one differential signal drives the light emitting element whereas the other differential signal drives a dummy of the light emitting element for good balance of the differential pair. Therefore, there arises a problem that power consumption is increased.
The following is a reference document.    [Document 1] Japanese Laid-open Patent Publication No. 11-4033.